Van hoek



Feb. 21, 1956 c. VAN HOEK 2,735,812

ELECTRODIALYZING APPARATUS WITH SUPPORTED MEMBRANES Filed March 10, 1953 2 Sheets-Sheet 1 Feb. 21, 1956 c. VAN HQEK 2,735,812

ELECTRODIALYZING APPARATUS WITH SUPPORTED MEMBRANES Filed March 10, 1953 2 Sheets-Sheet 2 United States Patent ELECTRODIALYZING APPARATUS WITH SUPPORTED MEMBRANES Cornelis van Hoek, Wassenaar, Netherlands, assignor to Nederlandse Centrale Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek, The Hague, Netherlands, a corporationof the Netherlands Application March 10, 1953, Serial No. 341,559 Claims priority, application Netherlands March 13, 1952 8 Claims. (Cl. 204-301) The present invention relates to electrodialyzing apparatus, whereby use is made of two or more membranes, through which ions are transported from the dialysis compartment to the adjacent compartments. Generally, the liquids in these adjacent compartments should be refreshed regularly. The simplest construction is the so-called 3-chamber apparatus, in which the anode compartment with the anode therein is separated .from the dialyzing compartment by a membrane and the dialyzing compartment is separated from the cathodecontaining compartment by a second membrane.

The electrical energy required for the desalting increases under otherwise similar conditions in proportion to the distance between the electrodes and with the electric resistance of the membranes. Consequently it is desirable to keep the distance between the electrodes and the membranes, and notably the mutual distance between the membranes, small. Under production conditions these distances should be fixed at determined values, otherwise locally difiering current densities may occur, which may disturb the satisfactory practice of the process. Care should be taken to prevent. the membranes from coming into contact with one another or with the electrodes.

Membranes having a sufliciently low electric resistance generally consist of thin, mechanically weak material, which is in itself not form-retaining, as for example regenerated cellulose.

Especially in technical apparatus in which membranes having large surface areas are used and in which the distances between electrodes and membranes and between the membranes should preferably be a few mm. at most, this makes high high demands upon the construction of the cells, since these membranes are as a rule elastic and plastically deformable and consequently must be retained in position and shape by special supporting devices. As a matter of course the support must not increase the resistance of the electrodialyzing compartment undesirably. At the same time no noticeable changes in shape should take place because of the difference in pressure in the various compartments at the working temperatures, also in case of prolonged use. Moreover, the supporting material should be chemically resistant to the rinsing liquids used, the composition of which varies with that of the dialysate. In the presence of chlorides or chlorine compounds in the dialysate, chlorine-resistant materials should be chosen for use in the anode compartment, for example of polyvinyl chloride or of a copolymer of vinylchloride and vinylidenechloride.

A drawback of many types of membrane supports is that the liquid is as a rule relatively stagnant at the edges of the support so that corroding substances formed at the electrode may increase in concentration and attack the edges of the membranes (so-called edge-effect).

It has been suggested, heretofore, to fit similar supporting frames in the electrode compartments and in 2,735,812 Patented Feb. 21, 1956 the middle compartments, between which the membranes are pinched, thus dividing a membrane having a large surface into a large number of small, free sections. This type of support has the disadvantages of reducing the active surface of the membrane and increasing the resistance to flow of the liquid passing through the various compartments which results in an increase of the power consumption. At the same time the above-mentioned edge effect occurs in dead corners along the edges of the support.

It has also been proposed to use a coaxial arrangement of cylindrical electrodes and membranes, whereby the membranes are preferably supported by tube-shaped fabrics of polyvinylidcnechloride or of a copolymer from vinylidenechloride and vinylchloride. On prolonged use or due to the development of pressure differences in the various compartments and also due to increases in temperature, these supporting devices are plastically deformed, whereby the membranes may come into contact either with each other or with the electrodes. Finding supporting material which does not have the abovementioned drawbacks is quite diflicult in practice.

It has proved possible to decrease the corrosive character of the anode rinsing liquid on electrodialyzing chloride-containing liquids, e. g. by aeration or by the addition of sulphite. Nevertheless, there should be no dead corners or compartments in which corroding substances may collect. Also the acid or alkali content of the rinsing liquids is restricted to special maximum limits and for this reason a good and even replenishment of the rinsing liquids is imperative. Too high acid or alkali concentration for instance, in special cases may cause disturbing deposits to occur on the membranes, so that it is desirable not to allow the liquid touching the electrodes and containing the electrolysis products formed at the electrode to come into contact with the membrane. Between two membranes mutually the liquid should be mixed as thoroughly as possible.

An object of the invention is to provide a novel construction of an electrodialysis compartment, in which the distances between the electrodes and the membranes and between the membranes are small and whereby the membranes are kept in their places by supporting devices, thus avoiding attack of the membranes by electrodialysis products formed at the electrodes and whereby a good mixing of the liquid in the space between two membranes is obtained.

According to the invention, supporting devices are mounted in the electrodialysis apparatus to support the deformable membranes at a short distance from the elec trodes or from other membranes and providing spaces for rinsing between a membrane and an electrode or between two membranes, which supporting devices are arranged to maintain the correct distance between a membrane and the adjacent electrode or between two membranes, and may consist of screens or sheets of insulating material having holes or perforations therein and providing contact points for engaging and supporting the membranes which are distributed over the whole surface of the screens or sheets.

In order to prevent attack of the membranes by electrolysis products formed at the electrodes, the screens between a membrane and an electrode should be such that they also act as diaphragms. For this purpose the screens may be corrugated sheets, the corrugations of which are about parallel to the direction of flow of the liquid which has been determined by inlet and outlet openings. The sheets should be provided with apertures so that the movement of the ions is not hampered but the mixing of the liquid which is in contact with the electrode with the liquid in contact with the membrane is counteracted to a high degree. The places at whichthe supports engage the membranes should not be long, uninterrupted lines in the current direction of the liquid, inasmuch as some attack at the edges of such a supporting line still occurs, with resulting weakening of the membrane. This objection does not arise if the places of contact between the supports and the membranes do not form long, continuous lines in the direction of fiow. For that reason perforations are also applied on the tops of the corrugations.

Fair result has been obtained by fitting a single corrugated perforated plate of polyvinylchloride between electrode and membrane and in contact with them. Mixing of the liquid currents is sufiiciently prevented with perforation holes having a diameter of the same order and size as half the wave form of a corrugation. The direction of the corrugations should preferably be about parallel to the direction of the liquid flow in the electrode compartment; so that the rinsing liquid flows in separated parallel channels which are only linked by the perforation openings and are bounded either by the electrode and the screen or by the membrane and the screen. In this way the liquid currents are only mixed to a limited extent.

Instead of the perforated corrugated plate a screen of corrugated fabric may be used with even better results. Preferably a fabric or gauze of thermoplastic material is used, in which the desired wave profile is produced by heating in wave-shaped moulds.

Attack of the anode membrane by chlorine is prevented during electrodialysis of sea-water by fitting two similar corrugated perforated sheets separated by a similar, fiat fabric, to prevent the corrugated plates from sliding together. In this way, a distance between electrode and membrane of not more than 2.5 mm. can be maintained.

Besides, in the above-mentioned simple wave profile, recesses may be applied on both sides, so that the membrane is not supported by the tops of the corrugations, but by a special pattern of points of support. The active membrane area is somewhat increased by this.

Also other shapes of the screen are possible, provided only the following conditions are fulfilled: the liquid which is in contact with the electrode should not mix too much with the liquid in contact with the membrane, the points or lines of support should not be thus sharp that they cause mechanical damage of the membrane and sufiicient support should be obtained to maintain the membranes in position, when in operation.

An advantage of supports as described above is that small measure discrepancies of the membrane can be eliminated evenly, thus without causing any folds. The occurrence of folds, causing dead spaces, should be prevented in any case, a condition which has been difficult to achieve with flexible membranes and the high pressure differences between dialysate and electrode compartment.

Supporting devices applied between two membranes should prevent the membranes from being pressed towards each other, or even touching each other when the pressure in the compartments is lower than that in the adjacent compartments.

The supporting device in these compartments need not be aligned with the supporting device in the electrode compartments.

Preferably the supports in the compartments between the membranes should not work as a diaphragm and should promote the mixture of the liquid in these compartments. For this purpose the perforated corrugated support device can be fitted in these compartments in such a way that the direction of the wave is not parallel to the direction of the liquid flow, determined by the location of the inlet and outlet openings, so that the liquid does not move in parallel directions, but is thoroughly mixed by the forced stream through the perforations.

If an electrodialyzing apparatus having'more than one compartment, and containing a large number of alternatively positive and negative ion-selective membranes between the electrodes, is constructed with the support according to the invention, the corrugated plates can advantageously be fitted in the electrode compartments in such a way that the longitudinal direction of the corrugations corresponds with the current direction, while in the intermediate dialysate compartments and rinsing compartments the corrugated plates form angles of 60 or with the current direction. There is no danger of the corrugated plates being pushed together on both sides of the membrane, and at the same time a good mixing of the contents of the latter dialysate compartments and rinsing compartments is guaranteed.

For a better understanding of the present invention, reference may be had to the accompanying drawings in which:

Figure l is a perspective view of a typical supporting device for the membrane of an electrodialyzing cell embodying the present invention;

Figure 2 is a perspective view of a composite supporting device or separator;

Figure 3 is a perspective view of a further modification of the supporting device or separator;

Figure 4 is a perspective view of the membranes, electrodes and supporting elements of a three-compartment dialyzing apparatus embodying the present invention;

Figure 5 is a perspective view of a dialyzing cell including an additional separator between the membranes;

Figure 6 is a schematic perspective view of a part of a dialyzing cell including a plurality of compartments; and

Figure 7 is a view in vertical section through a typical cell embodying the present invention and including separators arranged in the manner disclosed in Figure 5.

The supporting elements or separators for the membranes of the electrodialyzing apparatus are formed of a non-conductive material such as a sheet of plastic or thermoplastic fabric, having a large number of holes 2 therein and a plurality of wave troughs 3 and wave tops 4 forming corrugations in the sheet. The perforations may constitute as much as or more than half of the total area of the sheet. The membrane rests against the tops of the corrugations and is supported thereby.

Another type of support in accordance with the present invention includes two corrugated sheets 5 and 6, each similar to the support described above. 6 are disposed on opposite sides of a sheet of fabric 7 which serves to keep the corrugations from nesting together.

Still another form of separator may be a wave-like sheet similar to that shown in Figures 1 and 2 but provided with a plurality of protuberances or bumps 8 along the crests and troughs of the corrugations in the sheet.

Figures 4, 5, 6 and 7 illustrate the manner in which the supports 1, 5, 6, etc., are utilized in an electrodialyzing cell. In this cell, which is shown without the enclosure in Figures 4, 5 and 6, the anode 8 has a separator or support 9 adjacent to it for supportinga dialyzing membrane 10 in spaced relation to the anode. The opposite side of the middle compartment through which the solution to be treated flows is formed by the cathode membrane 11 which is supported by a support or separator 12 interposed between the membrane 11 and the cathode 13. The supports or. separators 9 and 12 may be like any of those disclosed in Figures 1 to 3.

The dialyzing cell shown in Figure 5 is similar to that shown in Figure 4 with the exception that another separator 14 is interposed between the membranes and is arranged with its corrugations at substantially a right angle to the direction of flow of the liquid through the intermediate chamber thereby to assure mixing of the liquid.

Figure 6 shows a multiple compartment apparatus having ion-selective membranes therein. Against the The sheets 5 and KJ. la- 4" anode 15 and cathode 16 are screens 17 and 18 having their corrugations parallel to the direction of liquid flow therein. In the dialyzing compartments between the membranes 19, 20 or 21, 22 and 23, 24 lie separators or supports having their corrugations arranged alternately at an angle of about 60 to the direction of fiow of the liquid in the compartments between the membranes.

Figure 7 illustrates a typical single cell dialyzing apparatus containing electrodes and a membrane and separator structure like that shown in Figure 5. The dialyzing apparatus includes a cell 25 formed of nonconductive materials such as glass or the like having outer side walls 26 and 27 with peripheral flanges 28 and 29 that are clamped together in sealing engagement with the edges of the interposed membranes and 11. A rim member 30 is interposed between the edges of the membranes to hold them in position and form the central compartment. The membranes are further positioned, as described above, by the interposed separator 14 and the separators 9 and 12 on opposite sides of them and in engagement with the anode 8 and the cathode 13, respectively. The rim 30 is provided at its opposite ends with an inlet 31 and an outlet 32 permitting circulation of the liquid therethrough and the rinsing cells on opposite sides of the membranes are also provided with suitable inlets 33 and 34 and outlets 35 and 36. It will be understood that the cell described above may be used to house the unit shown in Figure 4, or it may be enlarged to support a multiple cell unit like that shown in Figure 6 by appropriate arrangement of the spacer rims 30 to support the various membranes or diaphragms.

Example I For comparison two electrodialyses were carried out. In one test, a membrane support of free-bearing gauze, consisting of a copolymer of vinylidene chloride and vinylchloride was used, whereas in the other test, a corrugated support 1 of polyvinylchloride, resting against the electrode was used. The tests were conducted in a cylindrical electrodialysis compartment with a membrane surface of 2800 cm. and a current strength of 400 amp., that is, a current density of 143 mA./cm. The distances between the electrodes and the membranes and between the membranes each amounted to 3 mm. Concentrated whey of the following composition was used as a dialysate:

35% dry substance,

10% protein,

8% ash, of which 1.5% Cl, 17% lactose.

The rinsing liquids used were: anodically 0.15 N acid (principally sulphuric acid) and cathodically 0.2 N alkali. These rinsing liquids circulated at a velocity of approx. 20 litres per minute, while the dialysate circulated with 30 litres per minute. The active chlorine content of the anode rinsing liquid was kept at 50 mg. per litre at most by aerating the liquid in a chlorine desorption column with 100 litres air per minute.

After 20 working hours the anode membranes were tested both mechanically on strength and on chemical attack.

The mechanical strength of the membrane, used in combination with the wave-like screen, had not changed while the other had already noticeably fallen in strength.

The chemical attack of the membranes was compared by colouring with Turnbull blue and methylene blue. With both dyes the membrane, used in combination with the wave-like screen was practically not to be discerned from non-used membrane material. With the other membrane a very clear coloration was already noticeable, especially with methylene blue.

Example II Seawater was electrodialyzed in an apparatus of the type shown in Fig. 6, with distances between the membranes and electrodes of approx. 3 mm. and between the membranes mutually of approx. 2 mm. In one test, a membrane support of flat gauze formed of the above-mentioned copolymer was used, and in another test, supports 5 and 6 were used in the electrode compartments in a manner shown in Fig. 2, and in the other compartments according to Fig. 1 and sea-water was used as rinsing liquid. In the first test, the anode membrane broke down by attack after 7-10 working hours. In the second test, after 40 hours no deterioration of the quality of the membrane could be detected.

Example Ill Brackish water with a content of 1000 mg. Clper 1. was subjected to electrodialysis in an apparatus of the kind shown in Fig. 6 for such a period of time, that the outflowing water contained 300 mg. Cl' per 1. The total number of membranes amounted to 12. The support between these membranes consisted of corrugated perfd rated polyvinylchloride-sheets having a thickness of 0.3 mm., a weight height of 4 mm., an amplitude of 0.8 mm.; diameter of the perforations was 2.5 mm., with 1000 perforations per dm. of the flat sheet. The distance between the membranes amounted to 1.6 mm. The membrane surface was 800 cm. per membrane (20 (40 cm.). In the electrode compartments two corrugated plates were applied, separated by a flat fabric of gauze. The first corrugated plate along the electrode was mounted in such a way that the longitudinal direction of the corrugations coincided with the direction of flow of the liquid, and the second corrugated plate was disposed with its corrugations in a direction perpendicular to the corrugations on the first plate. In the other compartments only one corrugated plate was used per compartment, alternating at angles of 60 and 120 between current direction and longitudinal direction of the corrugations. The current density amounted to 2.5 rnA./cm. the fresh water production to 10 l./h. After a production of 1000 1. fresh water no deterioration of the quality of the membrane could be proved.

Example IV In an apparatus according to Fig. 6 seawater with a content of 18,000 mg. Cl/l. was subjected to electrodialysis for such a period of time that the outflowing water contained 300 mg. Cl-/l. The total number of mem branes amounted to 60. The support between these membranes consisted of corrugated polyvinylchloride-sheet having a thickness of 0.08 mm., a waveof 2 mm., an amplitude of 0.175 mm.; diameter of the perforation 2.2 mm., 1200 perforations per drn. flat sheet. The distance between the membranes amounted to 0.35 mm. The membranes had a surface of 800 cm. per membrane (20x40 cm.). In the electrode compartment two corrugated plates were fitted, separated by a flat fabric. These plates were fitted as described in Example III. In the other compartments only one corrugated plate was used per compartment, alternating at angles of 70 and 110 between current direction and longitudinal direction of the wave. The current density amounted to 2.5 mA./cm.'-, the fresh Water production to l l./h. After a production period of h. no deterioration of the quality of the membranes could be detected.

I claim:

1. In electrodialyzing apparatus comprising a chamber containing electrodes and flexible permeable membranes forming compartments therebetween and means for directing liquid through said compartments including an inlet for liquid at one end of the compartment and an outlet for liquid at the other end, the combination therewith of supporting members interposed between and engaging said membranes and said electrodes to maintain them in accurately spaced relation, said supporting members comprising sheets of insulating material having a multiplicity of perforations widely distributed therein, said sheets having corrugations therein to engage said electrodes and said membranes, said corrugations being interrupted to avoid uninterrupted line contact between said supporting members and the electrodes and membranes.

2. The apparatus set forth in claim 1 in which a supporting member between an electrode and an adjacent membrane has corrugations extending in the direction of flow of the liquid to reduce the mixing of liquid adjacent the electrode with the liquid adjacent to the membrane.

3. The apparatus set forth in claim 1 in which the corrugations on a supporting member between adjacent membranes are elongated and extend transversely to the direction of flow of the liquid to promote mixing of the liquid between the membranes.

4. The apparatus set forth in claim l in which the perforations are distributed substantially uniformly throughout the entire sheet.

5. Apparatus according to claim 1 wherein the combined areas of the perforations amounts to at least half of the area of the sheet.

6. Apparatus according to claim 1 wherein the supporting members consist of two perforated corrugated plates, separated by a flat foraminous sheet.

7. Multi-cell apparatus according to claim 1 wherein the wave direction of the corrugations of the supporting member of at least one cell is at an angle to the wave direction of the corrugations of the supporting member in the next cell and also at an angle to the direction of flow determined by the inlet and outlet of the cell.

8. In electrodialyzing apparatus comprising a charmher containing electrodes and flexible permeable membranes forming compartments therebetween and means for directing liquid through said compartments including an inlet for liquid at one end of one of said compartments and an outlet for liquid at the other end, the combination therewith of supporting members interposed between and engaging said membranes and said electrodes to maintain them in accurately spaced relation, said supporting members between said membranes and electrodes consisting of two perforated sheets having corrugations therein, said perforated sheets being separated by a flat foraminous sheet and wherein the supporting members between adjacent membranes consist of corrugated sheets, the wave directions of which are at an angle to the direction of flow of the liquid from the inlet to the outlet, the corrugations in the perforated sheets engaging said membranes being interrupted to avoid uninterrupted line contact between said supporting members and the membranes.

References Cited in the file of this patent UNITED STATES PATENTS 1,915,568 Gortner June 27, 1933 2,049,828 Stevens Aug. 4, 1936 2,225,024 Weber Dec. 17, 1940 2,664,395 Marchand Dec. 29, 1953 2,686,154 MacNeill Aug. 10, 1954 FOREIGN PATENTS 67,903 Netherlands May 17, 1951 

1. IN ELECTRODIALYZING APPARATUS COMPRISING A CHAMBER CONTAINING ELECTRODES AND FLEXIBLE PERMEABLE MEMBRANES FORMING COMPARTMENTS THEREBETWEEN AND MEANS FOR DIRECTING LIQUID THROUGH SAID COMPARTMENTS INCLUDING AN INLET FOR LIQUID AT ONE END OF THE COMPARTMENT AND AN OUTLET FOR LIQUID AT THE OTHER END, THE COMBINATION THEREWITH OF SUPPORTING MEMBERS INTERPOSED BETWEEN AND ENGAGING SAID MEMBRANES AND SAID ELECTRODES TO MAINTAIN THEM IN ACCURATELY SPACED RELATION, SAID SUPPORTNG MEMBERS COMPRISING SHEETS OF INSULATING MATERIAL HAVING A MULTIPLICITY OF PERFORATIONS WIDELY DISTRIBUTED THEREIN, SAID SHEETS HAVING CORRUGATIONS THEREIN TO ENGAGE SAID ELECTRODES AND SAID MEMBRANES, SAID CORRUGATIONS BEING INTERRUPTED TO AVOID UNINTERRUPTED LINE CONTACT BETWEEN SAID SUPPORTING MEMBERS AND THE ELECTRODES AND MEMBRANES. 